Task: Delete legacy ORE representations

Delete the legacy ORE representations for market data identity. No backwards compatibility: the cutover is hard.

• The columns series_type, metric, qualifier, ore_key, and currency_code are dropped from the schema. point_id stays: it carries the serialised oresmd point, per the observation task.
• The import classification table and the split_market_series_key identity use are deleted.
• The DB check constraints on identity are gone; validation lives in the oresmd layer.
• The tests that pin the old shape are updated; the build and the test suite are green.
• Nothing ORE-shaped is stored.

(Implementation strategy. Written when work starts; key decisions are distilled into the parent story's * Decisions at close, but the plan itself stays — it is the historical record of what we did.)

Manual QA scenarios (scaffolded via compass add test_scenario, run through the QA Validation Runner panel) that verify this task. Link new ones here as they're created; the scenario doc itself links back via its "Verifies task" field.